A conventional method is disclosed in Patent Literature 1 to facilitate formation of an insulating structure between devices to be formed on a Group-13 element nitride semiconductor layer. Specifically, ions are implanted into a Group-13 element nitride semiconductor layer in an area surrounding an element region (hereinafter referred to as an element isolation region) to form a modified region therein. An oxidation reaction is promoted in the modified region during an oxidation treatment. This facilitates formation of an insulating oxide film in the element isolation region, thereby making an insulating structure between devices with ease. Although such a method facilitates the formation of an insulating oxide film in the element isolation region, it also causes an insulating oxide film to be formed partially in the element region due to thermal oxidation. This results in increased surface unevenness. There is a difficulty in that it causes a reduction in mobility, which leads to degradation in performance of a switching device to be formed in the element region.
In another configuration in which devices are formed using a GaN layer and an AlGaN layer formed over a buffer layer provided on a substrate, a trench isolation structure is formed between the devices. The trench isolation structure extends from the surface of the AlGaN layer to the substrate or to a position near the substrate. This requires a deeply etched trench, which results in an increase in manufacturing process time and thereby an increase in manufacturing cost. This also necessitates a thick resist film or a metal mask as a mask material for use during the etching to allow for a deep trench, posing difficulty in achieving high integration in a semiconductor device.
As a solution to the difficulty described above, an insulating film is formed in an element isolation region using deposition. In the case of forming an insulating film using deposition, a semiconductor device is manufactured in a process, for example, as described below.
An AlGaN layer is formed on a GaN layer. Then, an element isolation region, which is an area surrounding an element region, is lowered to a height below the element region to provide a mesa structure. A silicon oxide film is then deposited on the surfaces of the element region and the element isolation region to provide an insulating film. Subsequently, an opening is formed, using a desired mask, in the silicon oxide film at a position where a gate electrode is to be placed. The opening extends through the silicon oxide film to the AlGaN layer to form a recessed part. A gate electrode is placed over a gate insulating film in the recessed part. Further openings are formed, using a desired mask, in the silicon oxide film at positions where a source electrode and a drain electrode are to be placed, that is, on opposing sides of the gate electrode. The openings extend through the silicon oxide film to the AlGaN layer to form recessed parts. The source electrode and the drain electrode are then placed in the recessed parts. A semiconductor device having an insulating film formed using deposition in an element isolation region is manufactured in this manner. A silicon nitride film, in place of a silicon oxide film, may be employed as an insulating film to be formed on the surfaces of the element region and the element isolation region.